Many microwave electronic systems operate in a pulsed-RF mode. Radar systems and time-division multiple access (“TDMA”) wireless communications systems are examples of systems that operate in a pulsed-RF mode. Radar systems typically operate with signals in the range of several gigahertz (“GHz”) to tens of GHz and use devices such as antennas, amplifiers, transmit-receive modules, and frequency converters (“mixers”). TDMA wireless communication systems typically operate below about 6 GHz, and use similar electronic devices as are used in radar systems. However, most electronic devices are tested under continuous wave (“CW”) conditions. That is, the electronic device that is being tested, which is commonly called a “device under test” or “DUT,” is stimulated with a CW signal, and the response of the signal is measured with a receiver, such as a signal analyzer or a network analyzer.
Some electronic devices behave differently when stimulated with a pulsed-RF signal, rather than a CW signal. Bias changes during the RF pulse can affect the radio-frequency (“RF”) performance of the device. Overshoot, ringing or droop (gain reduction during the latter part of the RF pulse typically due to self heating) that does not occur when the device is tested with a CW signal can result when using a pulsed signal. In other instances, a CW signal might destroy the DUT. For example, performing a CW wafer test (i.e. on DUTs that have not been separated from the wafer on which they were fabricated) might destroy a DUT that is not adequately heat sunk. Other DUTs might not be designed to operate in a CW mode, such as high-power amplifiers used in radar systems. More information on testing DUTs using pulsed-RF measurement techniques is found in Pulsed-RF S-Parameter Measurements Using a VNA by David Ballo, AGILENT TECHNOLOGIES, INC. (Oct. 13, 2004), the disclosure of which is hereby incorporated in its entirety for all purposes.
There are two conventional techniques for pulsed-RF testing of DUTs. The first technique is commonly called the “wide-band” synchronous pulsed measurement technique. A receiver with a relatively wide output bandwidth, specifically an output bandwidth sufficient to allow the receiver to capture all or essentially all of the RF pulse energy, is used. To measure a characteristic of interest of the RF pulse, such as rise time, the RF pulse rise time must be longer than 1/BW. The minimum RF pulse duration measurable using this technique is limited by the maximum bandwidth of the receiver being used.
Wide-band pulsed-RF measurements are synchronous. That is, the receiver is synchronized with the incoming RF pulses and knows when to capture (i.e. measure) the RF pulse energy. This requires a trigger signal, which for periodic RF pulses can be internally generated by the receiver, such as a MODEL 8510™ network analyzer manufactured by AGILENT TECHNOLOGIES, INC., of Palo Alto, Calif., or the trigger signal is provided to the receiver from an external source, such as a pulse generator. Wide-band pulsed-RF measurements are desirable because the dynamic range is independent of duty cycle; however, the dynamic range is limited by the wide IF bandwidth.
There is a lower limit of measurable RF pulse widths. As the RF pulse width becomes shorter, the spectral energy of the RF pulse in the frequency domain spreads out. RF pulses of short duration may have spectral content that falls outside the IF bandwidth. If a significant amount of energy is outside of the bandwidth of the receiver, the receiver cannot accurately represent and measure the RF pulse response of the DUT.
The second pulsed-RF technique is commonly called the narrow-band asynchronous pulsed-RF measurement technique (“narrow-band RF pulse detection”). Narrow-band RF pulse detection is used when enough of the RF pulse spectrum is outside the bandwidth of the receiver so that wideband detection cannot be used. With this technique, everything except the central frequency component (“center tone”) of the pulsed-RF spectrum is filtered out by the receiver. A relatively narrow (compared to the spectrum of the RF pulse) IF filter is used, and an arbitrarily narrow RF pulse may be measured. The narrow IF filter measures the center tone of the RF pulse spectrum (which represents the frequency of the RF carrier). After filtering, narrow-band RF pulse detection is similar to a CW measurement, which receivers handle very well.
With narrow-band RF pulse detection, the sample periods of the analyzer are not synchronized with the incoming RF pulses; therefore, no pulse trigger is required. This is why this technique is often called asynchronous acquisition mode. An advantage of using narrow-band RF pulse detection is that there is no lower RF pulse-width limit, since no matter how broad the RF pulse spectrum is, most of it is filtered away, leaving only the center tone of the DUT's RF pulse response spectrum.
Unfortunately, the dynamic range of the measurement is a function of duty cycle. As the RF pulse duty cycle drops, the energy in the central tone drops while the noise power stays constant. Thus, as the duty cycle of the RF pulses decreases (i.e. longer time between RF pulses), the average power of the RF pulses gets smaller, which degrades the signal-to-noise ratio. The effect is often called “pulse desensitization.” This causes the dynamic range of narrow-band asynchronous RF pulse detection to degrade by 20*log (duty cycle). The narrow-band asynchronous RF pulse measurement technique is sometimes called a high pulse repetition frequency (“PRF”)” technique, since the PRF is normally much greater than the IF bandwidth in order to maintain good dynamic range.
Therefore, methods of measuring devices using pulsed-RF that avoid the disadvantages described above are desirable.